I cannot find the MSI format specification. It says that its an open standard, but I don\'t see any documentation - just bits and pieces of information here and there.
Microsoft has published the MSI file format details. You can download the specifications for the various versions here: [MS-CFB]: Compound File Binary File Format
The latest version at the time of this writing is 9.0, released on 9/12/2018. You can download it as a PDF here.